usb_fifo_open() curr_cpd single-owner check is unlocked — concurrent opens of the same ugen endpoint bypass EBUSY and yield two live owners on one fifo

Summary

'we don't need any locks' NULL check at :701 vs locked publish at :736 with the f_open lock round-trip in between: two racing opens both pass, both run f_open/f_close lifecycles, curr_cpd clobbered by the last writer - the earlier cpd's dtor then skips close while both fds stay usable on one fifo. Defeats the single-owner invariant the fifo engine and FIOASYNC state depend on; distinct entry into the DF-5287 concurrent-setup sink plus owner clobbering and a leaked fifo when create's unlocked slot search races. Atomic check-and-set fix.